内容简介：
"Mark hand me that test tube, will you, please?" The lad who had made the request looked over at his companion, a boy of about his own age, who was on the other side of the laboratory table. "The big one, or the small one?" questioned Mark Sampson. "The large one," answered Jack Darrow. "I want to put plenty of the chemical in this time and give it a good try." "Now be careful, Jack. You know what happened the last time." "You mean what nearly happened. The tube burst, but we didn't get hurt. I have to laugh when I think of the way you ducked under the table. Ha, ha It was awfully funny " "Humph Maybe you think so, but I don't," responded Mark with rather a serious air. "I noticed that you got behind a chair."